Type C viruses isolated from most transformed clones of a 10T1/2 cell line, established from the low leukemic strain of mouse C3H, were found to differ from viruses obtained from non-transformed clones in that only the former replicate efficiently on the cell of origin, cause the appearance of the GIX antigen on the surface of infected cells and the formation of syncytia in contact with XC cells. The XC-phenotype, that is the ability of type C viruses to cause syncytial formation in the XC test, has been analyzed and identified as a host range function. Activation and cocultivation experiments with a chemically transformed cell line from 129 mice have led to the isolation of ecotropic MuLV.